Photoelectrophoretic imaging with fixing on a separate electrode

ABSTRACT

A photoelectrophoretic imaging system wherein electrically photosenstive pigment particles are dispersed in an insulating liquid exposed to a light image while subjected to an electrical field causing particle migration in image configuration. The particle pigment image is fixed by utilizing resinous particles which can be preferentially migrated to the image-bearing surface. Application of solvent or heat fixes the images.

United States Patent [19] Wells et al.

[111 3,820,987 [4 June 2 8, 1974 PHOTOELECTROPI-IORETIC IMAGING WITH FIXING ON A SEPARATE ELECTRODE [75] Inventors: John B. Wells, Rochester; Addison C. Sheckler, Cato, both of NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

22 Filed: Dec. 6, 1972 [21 Appl. No.: 312,532

[52] US. Cl 96/l.3, 96/1 PE, 96/1.4,

3,241,998 3/1966 Oliphant 117/37 LE 3,384,565 5/1968 Tulagh et a1 96/1.3 X 3,547,631 12/1970 Weglein 96/1 R X 3,648,607 3/1972 Gundlach... 6/1 PS X 3,655,370 4/1972 Carriera et a1. 96/1 PE 3,669,859 6/1972 Merrill 96/1 R X 3,705,797 12/1972 Mihajlov et a1. 96/1 PE Primary Examiner-Roland E. Martin, Jr. Attorney, Agent, or Firm-lames .1. Ralabate; David C.

Petre; Richard A. Tomlin 5 7] ABSTRACT A photoelectrophoretic imaging system wherein electrically photosenstive pigment particles are dispersed in an insulating liquid exposed to a light image while subjected to an electrical .field causing particle migration in image configuration. The particle pigment image is fixed by utilizing resinous particles which can be preferentially migrated to the image-bearing surface. Application of solvent or heat fixes the images.

8 Claims, 3' Drawing Figures 1 PHOTOELECTROPHORETIC IMAGING WITH FIXING ON A SEPARATE ELECTRODE BACKGROUND OF THE INVENTION This invention relates in general to imaging systems and more particularly to an improved photoelectrophoretic imaging system.

There has been recently developed a photoelectrophoretic imaging system capable of producing images of one or more colors which utilizes electrically photosensitive particles. This process is disclosed and claimed in U.S. Pat. Nos. 3,384,488 and 3,384,565 to V. Tulagin and L. Carreira; 3,383,993 to S. Yeh and 3,384,566 to H. E. Clark, all issued May 2], 1968, the disclosures of which are incorporated herein by reference. In such an imaging system, colored lightabsorbing particles are suspended in a non-conductive carrier liquid. The suspension is placed between electrodes, subjected to a potential difference and exposed to an image of radiation to which the particles respond. As these steps are completed, selective particle migration takes place in image configuration providing an image on at'least one of the electrodes. Normally, a positive image is formed on one electrode and a negative image is formed on the opposite electrode. An essential component of the system is the suspended particles which must be electrically photosensitive and which apparently undergo a net change in charge po larity upon exposure to activating electromagnetic radiation, through interaction with one of the electrodes. In a monochromatic system, all of the particles may be of one color and they may respond panchromatically or have a response to wavelengths wtihin the range of wavelengths of the imagewise radiation. Conversely, for polychromatic systems, particles are chosen which have a response to a limited range of wavelengths. Particles of more than one color are used, each particle of a given color having a response which does not substantially overlap the response of the particles of other colors. The separate responses are required to achieve color separation. For subtractive full color imaging, it is desirable to use cyan colored particles responsive mainly to red light, magenta colored particles responsive mainly to green light and yellow particles responsive mainly to blue light. Particles used in this system must have both intense and pure colors and must be highly photosensitive. Particles meeting the above requirements are generally pigment materials which are insoluble in most liquids and do not readily soften on heating making image fixing difficult.

After the above described exposure and particle migration steps are completed, the electrodes are separated and the carrier liquid runs off or is evaporated. This leaves images on one or both electrodes made up of selectively deposited particles of pigment. The images are at this time fragile and easily damaged. The carrier liquid may contain a small proportion of a dissolved wax or other binder which on evaporation of the carrier liquid would serve to bind the particles together. However, if more than a very small amount of binder material is dissolved, undersirable interference with the imaging process takes place.

It has been suggested that a transparent sheet be laminated over the images, or a transparent binder material be sprayed over the images to form a protective coating. While, when carefully done, these techniques will protect the image, the image may be damaged during application of the protective material. Other methods of fixing images use heat or solvent tackifiable layers which are rendered tacky and contacted to the image. Those techniques are disclosed in copending applications Ser. Nos. 459,860 filed May 28, 1965, now abandoned; 677,706, now abandoned, and 677,707 filed Oct. 24, 1967. These techniques suffer from the same defects mentioned above.

Also, when it is desired totransfer the image from an electrode to a receiving sheet, the dangers of damaging an unfixed image is high. Thus, there is a continuing need for better systems for fixing particulate images.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a photoelectrophoretic imaging system which over comes the above-noted disadvantages.

It is another object of this invention to provide a method of protecting a photoelectrophoretically formed image from damage.

It is another object of this invention to provide a photoelectrophoretic imaging system which has a relatively simple image fixing step. t

It is another object of this invention to provide a photoelectrophoretic imaging system which uses relatively little fixing material and is relatively inexpensive.

It is another object of this invention to provide a photoelectrophoretic imaging system capable of producing images having a variety of finishes.

The above objects and others are accomplished in accordance with this invention by providing an imaging suspension of electrically photosensitive particles in an insulating liquid. Finely divided particles of a resinous material are incorporated into the suspension with the electrically photosensitive particles. The imaging suspension is then exposed to a pattern of radiation to which the photosensitive particles respond and an electrical field is applied across the suspension. An image is formed by the migration of photosensitive particles in light struck areas. By careful selection of materials and system polarities, the particles of resin can be made to deposit with or on the image which it is desired to fix. Application of heat or solvent softens the resinous material allowing the pigment particles to become embedded in the resinous material. Cooling or removing the solvent allows the resinous material to harden trapping the particles therein and firmly attaching the image to the substrate. The image substrate may be the member on which the image was formed or to which the image has been transferred.

ln a particularly preferred embodiment for full color subtractive imaging, an imaging suspension without the resinous particles is placed on a transparent conductive substrate called an injecting electrode. A second electrode in roller form having an insulating surface and called a blocking electrode because the insulating layer tends to block or reduce charge exchange is placed in contact with the free surface of the suspension. Field is applied causing the suspended electrically photosensitive particles to be driven to the surface of the transparent conductive electrode. The suspension is exposed to imagewise radiation through the transparent electrode while the roller electrode traverses the suspension. Particles which have been exposed to sufficient radiation to which they are sensitive migrate to the roller electrode and form a negative image thereon leaving a positive image on the surface of the transparent electrode. In order to remove those particles remaining behind which should have migrated during the first imaging pass, a second roller pass is used. The second roller pass clears up the background and improves color. Since the image desired is the full color image remaining on the transparent conductive electrode, this image is transferred to, for example, paper. The particulate resin material is suspended in a liquid which is compatible with the imaging suspension carrier liquid and added just prior to the transfer step. The transfer step is aided by electrical field application which provides the dual function of causing the particles forming the image to migrate through the liquid to the image receiving member and drawing the resin particles out of the liquid to deposit with the transferred image particles on the receiving member. The image is then fixed by heating or by solvent application to the receiver member.

Where the image is to remain on the transparent conductive substrate, such as when a material like aluminized polyester film is used as the transparent conductive substrate, the particulate resin material is preferably added just before the second imaging pass. The second roller can be used to introduce the resin particles into the system and to drive the particles down to the image bearing surface while removing unwanted particles of photosensitive pigment from the image. The second roller pass is made at the same polarity and with the same illumination as is used during the first imaging pass.

For monochrome imaging one imaging pass is conventionally used with the particles which migrate to the roller electrode forming the desired image. This image is a negative of the input image. In this instance the resinous material is preferably added to the original imaging suspension; or, if the image is to be transferred, it is preferred to add the fixing material just priorto or during transfer.

In general, it is desirable to introduce the particulate resinous fixing material into the system as late in the process as possible. This is preferred to keep particle interaction to a minimum and to prevent the particles from interfering with the migration of electrically photosensitive particles. The resinous particles may also be introduced into the system after final image transfer is complete. In order to draw the particles to the image, it is, however, necessary to provide an additional source of potential.

The success of the process depends on selecting resin materials which are strongly attracted by an electrode and will remain on the electrode until the various subsequent process steps are completed. A single test method has been found which can predict whether a particular resin will work in the system.

The test method used is to disperse about parts by weight of the resinous material in the form of finely divided particles in about 100 parts by weight of an insulating carrier liquid. The suspension is coated onto a conductive plate to a thickness of about 4 microns. A roller electrode having an insulating layer onits surface is roller across the suspension while an electrical field is applied between the conductive plate and the conductive core of the roller electrode. This potential should be below about 2,500 volts to avoid corona charging of the particles. It is noted to which electrode that is the positive or negative electrode the particles migrate. In general, thermoplastic resin materials are preferred. It is also desirable that the resinous material not be highly soluble in the carrier liquid and that it be transparent so as not to interfere with the color of the final image although dyed material may be used to alter the color of the formed image if desired particularly for monochrome images. It is also preferred that the heatfixable material soften at a temperature of between about 200 and 300F.

Typical resinous materials include vinyl polymers, polystyrenes, polyethylenes, polypropylenes, polyimides, polyamides, polyesters, polycarbonates, phenoxies, fluorocarbons and mixtures and copolymers thereof where applicable. In addition, resinous materials which have been found particularly suitable include melamine formaldehyde, urea formaldehyde, and phenolformaldehyde type resins, methyl methacrylate, polyvinylchloride, fluoropolymer, polyvinylidene chloride and ethyl cellulose. Fluoropolymer B, a thermoplastic fluorocarbon copolymer available from Du- Pont is preferred because it is strongly attracted by a positive electrode and is readily softened by radiant heating providing a well-fixed final image.

The photosensitive particles may comprise any suitable electrically photosensitive particles. Typical materials include finely divided particles such as those listed in U.S. Pat. No. 3,384,488 issued May 21, 1968 to V. Tulagin and L. Carreira. Typical particles include finely-divided particles of organic pigments such as quinacridones, carboxamides, carboxanalides, triazines, benzopyrrocolines, anthraquinones, azos, pyrenes, phthalocyanines both metal containing and metal-free; and inorganic materials, such as cadmium sulfide, cadmium sulfoselenide, zinc oxide, zinc sulfide, sulphur, selenium, mercuric sulfide, lead oxide, lead sulfide, cadmium selenide, titanium dioxide, indium trioxide and mixtures thereof. The particles may be of more than one component and may be dye sensitized to alter their spectral response. The X-form of metal-free phthalocyanine as shown in U.S. Pat. No. 3,357,989 to J. F. Byrne et al. is preferred for monochrome imaging because of its high response. For full color subtractive imaging, a mixture of about equal parts of metal-free phthalocyanine; the barium salt of Watchung Red B l- (4'-methyl-5 -chloroazobenzene-2'-sulfonic acid )-2- hydroxy-3-naphthoic acid C.I. No. 15865; and, a yellow pigment N2-pyridyl-8, l 3-dioxdinaphtho-( 2, lb;2',3-d)-furan-6-carboxamide prepared as shown in U.S. Pat. No. 3,477,922 is preferred because of its high sensitivity and excellent color separation capability.

It is desirable to use electrically photosensitive particles which are relatively small in size because smaller particles produce more stable suspensions with the carrier liquid and are capable of producing images of higher resolution than would be possible with particles of larger sizes. Thus, it is preferred that the photoresponsive particles be less than one micron in size al though particles of up to five microns may readily be used. Larger particle sizes tend to form less stable suspensions and cause loss of resolution. The acceptable resin material size range will depend on whether the resin is dispersed with the imaging suspension or in a separate resin suspension. Where the particles of resin are placed in the imaging suspension, they should be of a size similar to the photosensitive particles so as to provide a stable suspension and not interfere with photosensitive particle migration. Where a separate resin suspension is utilized, relatively larger particle sizes may be tolerated. Particles having a cross section of microns or greater may be used.

The imaging suspension and the resin suspension where a separate resin suspension is used may initially be coated on the various electrodes or transfer members. Typical coating methods include roller application, dip coating, spraying, electrophoretic plating, pouring or brushing.

The carrier for the imaging suspension and the resin suspension where applicable may comprise any suitable insulating material which may be liquid or it may be a solid which may be converted to a liquid at the time of particle migration. Typical insulating materials include decane, dodecane, N-tetradecane, kerosene, molten paraffin, molten beeswax or other molten thermoplastic material, mineral oil, silicone oils such as dimethyl polysiloxane and fluorinated hydrocarbons. Sohio Odorless Solvent 3454 (a mixture of kerosene fractions) is preferred because it is an excellent insulator and evaporates readily.

The amount of photosensitive and resin particles dispersed in the imaging suspension may vary over a wide range. Where the imaging suspension is used to provide the fixing material resin too, only as little as 2 parts of resinous material by weight based on 100 parts by weight carrier liquid may be used, up to about parts by weight may be used. The photosensitive particle content can range from about 2 to about 40 parts by weight based on I00 parts by weight carrier liquid. The range permissible will vary depending on how stable the suspension can be made, the sensitivity of the photosensitive ingredient, the operating conditions and other factors.

Although it is preferred to use a conductive electrode and an electrode having an insulating surface, the system will operate with both electrodes having insulating surfaces or both electrodes being conductive. It is preferred, however, to use a conductive electrode which allows ease of charge exchange with the photosensitive pigments and an electrode having an insulating layer which retards charge exchange preventing particle oscillation in the system and to help support the relatively high fields used in the process. A field of at least about 300 volts per mil across the imaging suspension is required to form images. Much higher voltages are routinely used however, for example, in the apparatus as shown in the drawing from 2,000 to 7,000 volts may be used. To further increase field strengthacross the imaging suspension, the electrodes are brought into virtual contact with a gap of up to about one mil used, Larger spacings cause loss of resolution and decreases the color separation capability of the polychrome system.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of this improved photoelectrophoretic imaging process and apparatus will become apparent upon consideration of the following detailed disclosure of the invention, especially when taken in conjunction with the accompanying drawings wherein:

FIG. 1 shows a schematic side view of a simple exemplary system for carrying out the process of this invention to produce monochrome images.

FIG. 2 shows a schematic side view of a simple exemplary system for carrying out the process of this invention to produce full color images.

The various elements and materials have been purposely distorted in size to aid understanding. For example, the liquid layer thickness and suspended particle sizes have been grossly enlarged.

Referring now to FIG. IA, there is seen a transparent electrode generally designated 1 which in this exemplary instance is made up of a layer of optically transparent glass 3 overcoated with a thin optically transparent layer of tin oxide, commercially available under the name NESA glassThis electrode is referred to as the injecting electrode.

Coated on the surface of electrode 1 is a thin layer of an imaging suspension generally designated 7 which comprises electrically photosensitive particles 8 dispersed in an insulating carrier liquid 9.

A second electrode 10 is provided which in this exemplary instance comprises a conductive aluminum roller 12 having a 2 mil Mylar polyester sheet 13 taped I on. The conductive center 12 of roller 10 and electrode 1 are connected to a source of do potential 11, the injecting electrode is also connected to ground.

In operation imagewise radiation 16 is used to expose imaging suspension 7. With power source 11 activated roller 10 is caused to roll across imaging suspension 7. As roller 10 traverses suspension 7 at the high potentials used, there is a corona generated between roller 10 and suspension 7 ahead of the area of contact which causes the photosensitive particles 8 to deposit substantially uniformly on surface 5. The imagewise radiation 16 causes particles responsive thereto to exchange charge and migrate to blocking electrode surface 13.

. On completion of roller traverse, a positive image is found adhering to the surface of electrode LA negative image is formed on the blocking electrode. For monochrome imaging it is preferred to use the image formed on the surface of. the blocking electrode 10 since it has less background than the image left behind on electrode 1. In this instance, a silver halide negative is used resulting in a positive image on the blocking roller electrode. it

Referring now to FIG. 1B, electrode 10 having on its surface particulate image 8 is brought into contact with roller 18 which applies a suspension of resinous particles 20 in an insulating liquid 21 to surface 13. A field is applied between conductive center 23 of roller 18 and conductive center 12 of roller 10 which cause particles of resin to deposit on surface 13 and particles 8. Application of heat from radiant energy source 27 fuses the image to the polyester sheet forming the final image.

Referring now to FIG. 2, electrode 41 is similar to electrode I discussed in connection with FIG. 1A. Im aging suspension 47 comprises electrically photosensitive particles 48 dispersed in carrier liquid 49.

The photosensitive particles 48 here, however, are a mixture of yellow, cyan and magenta particles sensitive mainly to blue, red and green light respectively. Full made by traversing suspension 49 with rollers 33 and 34 with field applied and exposure to radiation 56 continued.

Roller 37 is used to apply the suspension 40 of resinous particles in an insulating liquid to the surface of roller 39. Roller 39 bearing resinous suspension 40 is then rolled across electrode 41 with field applied. The field is of the opposite polarity as that used on electrodes 33 and 34. Electrode 39 picks up the image remaining on surface 45. This transfer may be aided by uniformly illuminating the image with radiation to which the particles respond. The image which is transferred to roller 39 has resinous particles mixed therein. The image is then readily fixed by heat or solvent application depending on the type of resin used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following Examples further specifically illustrate the improved photoelectrophoretic imaging system provided by this invention. Parts and percentages are by weight unless otherwise indicated. The following Examples are intended to illustrate various preferred embodiments of the present invention. All of the Examples are carried out in apparatus of the general type illustrated in the Figures. A 500 watt quartz iodine light source is used to illuminate a black and white negative or Kodachrome transparency, as applicable, the image being projected by a lens through the injecting electrode which is a tin oxide coated glass.

A source of high potential is connected to the blocking electrode and transfer electrode cores which are conductive steel rollers about 3 /2 inches in diameter. A 2 mil Mylar polyester sheet is wrapped around the blocking electrode rollers. A paper sheet is placed over the transfer roller where used to receive images. The

imaging suspension is coated on to the injecting electrode to a thickness of about 4 microns. The roller electrode is rolled across the imaging suspension at a rate of about 2 inches per second with field applied while the suspension is exposed to the images.

EXAMPLE I About 1 part of the x-form of metal-free phthalocyanine as described in US. Pat. No. 3,357,989 and about 1 part of polyvinylchloride available as GEON 121 available from Goodrich are dispersed in about parts of Sohio Odorless Solvent 3454. The mixture is milled until all particles are less than about one micron in cross section and a uniform dispersion is formed. The suspension is coated to a thickness of about four microns onto the conductive surface of a NESA plate. Roller traverse is made with a potential of about 3,000 volts applied between the center of the roller electrode and the NESA glass surface. The blocking electrode is positive with respect to the NESA electrode. Illumination is made through a negative black and white transparency. On completion of roller traverse, a positive cyan image is found adhering to the Mylar polyester surface. The image contains resinous particles. The image is then heated by radiant fusing. A well fixed image is thus formed on the surface of the Mylar sheet which resists abrasion.

EXAMPLE II An imaging suspension is made as in Example I but omitting the resin. An image is formed on the blocking EXAMPLE III An imaging suspension is made as in Example I but omitting the resin. An image is formed as in Example I. The blocking roller electrode with the image adhering to it is then contacted with a paper covered transfer roller having a 5 micron layer of a suspension of about 1 part of polyvinylchloride (GEON 121) from Goodrich in about 20 parts of Sohio 3454. A potential of about 3,000 volts is applied, the transfer roller being negative with respect to the blocking electrode, which transfers the image from the blocking electrode to the paper. The image is fused as in Example I.

EXAMPLE IV An imaging suspension is made up by dispersing about 0.75 grams of a magenta pigment, Watchung Red B, a barium salt of 1-(4'-methyl-5'- chloroazobenzene-2-sulfonic acid)-2-hydroxy-3- naphthoic acid, C.I. No. 15865; about 1.2 grams of a yellow pigment, N-2-pyridyl-8,l3-dioxodinaphtho- (2,l-b;2,3-d) furan-6-carboxamide, and about 1.8 grams of a cyan pigment, Monolite Fast Blue 6.8., the alpha form of metal-free phthalocyanine, C.l. No. 74100 and about 2.0 grams of Fluoropolymer B in 40 milliliters of Sohio 3454 and milled as in Example I. The negative black and white transparency used in the previous Examples is replaced with a full color Kodachrome transparency. Two imaging passes are made by traversing the imaging suspension coated to a thickness of about 6 microns on the NESA glass surface with a potential of about 3,500 volts applied between the blocking electrode centers and the NESA glass surface. The rollers are held at a negative potential with respect to the NESA surface. The'image of photosensitive particles and resin material is then transferred by traversing the image with a paper covered roller and applying a potential of about 3,500 volts between the conductive center of the transfer roller and the NESA surface. The roller is made positive with respect to the NESA surface. The transferred image is then fixed by heating either by radiant or heating by thermal contact with a heated plate.

EXAMPLE V A polychrome imaging suspension is made as in Example IV- omitting the resin. A first imaging pass is made as in Example IV. The second imaging pass is made using a roller coated with about a 4 micron layer of about I part of Fluoropolymer B resin in about 20 parts of Sohio 3454. The second roller pass deposits resin particles on the image formed on the NESA glass. The image and resin particles are then transferred using a paper covered transfer roller held at a potential of about 4,000 volts. Roller polarities are the same as in Example IV. The image is then fixed by exposure to methyl ethyl ketone vapors.

EXAMPLE VI EXAMPLE VII The experiment of Example V is repeated except that the transfer roller is not coated with a resin suspension. After transfer the roller is contacted to a similar roller having about a 6 micron layer coating of the resin suspension of Example VI on it. The resin particles are caused to deposit on the image bearing transfer roller by application of a field of about 2,000 volts with the transfer roller being positive with respect to the resin suspension bearing roller. The image is fixed by radiant heating.

EXAMPLE VIII The experiment of Example IV is repeated except that the resin material is replaced by melamine formaldehyde available as Radiant Clear Resin 2-1226. Image fixing is accomplished by exposure to methylene chloride vapors.

EXAMPLE IX The experiment of Example III is repeated except that the resin material is replaced by Fluoropolymer B. Image fixing is accomplished as in Example VIII by exposure to methyl ethyl keytone vapors.

Various specific components and proportions have been described in the above description of the preferred embodiments, other materials and proportions may be used. In addition, other materials may be added to enhance, synergize or otherwise modify their properties. For example, the pigments may be dye sensitized to alter their spectral response. Further, the pigments may be of more than one component where desired.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon reading the present disclosure. These are intended to be encompassed within the scope of this invention.

What is claimed is: V

l. The method of photoelectrophoretic imaging which comprises the steps of:'

a. providing a layer of an imaging suspension on a first substantially transparent electrode, said imaging suspension comprising electrically photosensitive particles of at least two differing colors dispersed in a substantially insulating carrier liquid, said particles of each color having a spectral response which does not substantially overlap the spectral response of particles of a differing color;

b. subjecting said suspension to an electrical field of a first polarity applied between said first electrode and a second electrode while substantially simultaneously exposing said imaging suspension to a pattern of radiation containing wavelengths of radiation to which particles of at least two colors are re sponsive until an image is formed on said first electrode;

c. contacting said image on said first electrode with a third electrodehaving an image receiving surface and coated thereon a suspension of resinousparticles'in an insulating liquid; I

d. applying an electrical field of a second polarity across said suspension and said image between said third-electrode and said first electrode until at least a portion of said image is transferred to said image receiving surface on said third electrode; and,

e. softening said resinous particles on said image receiving surface to fix said image.

2. The method of claim 1 wherein step (b) is repeated at least one additional time prior to step (c).

3. The methodof claim 1 wherein said softening is accomplished by application of heat.

4. The method of claim 1 wherein said accomplished by application of solvent.

5. The method of claim 1 wherein said resinous particles comprise polyvinyl chloride.

6. The method of claim 1 wherein said resinous particles comprise a thermoplastic fluorocarbon copolymer.

softening is cles comprise methylmethacrylate. 

2. The method of claim 1 wherein step (b) is repeated at least one additional time prior to step (c).
 3. The method of claim 1 wherein said softening is accomplished by application of heat.
 4. The method of claim 1 wherein said softening is accomplished by application of solvent.
 5. The method of claim 1 wherein said resinous particles comprise polyvinyl chloride.
 6. The method of claim 1 wherein said resinous particles comprise a thermoplastic fluorocarbon copolymer.
 7. The method of claim 1 wherein said resinous particles comprise melamine formaldehyde.
 8. The method of claim 1 wherein said resinous particles comprise methylmethacrylate. 